1. Field of the Invention
This invention relates to a gas turbine combustor for use in a gas turbine power generating system and the like, and in particular to a gas turbine combustor provided with a catalyst which suppresses the generation of nitrogen oxides (NOx) as environmental pollutants.
2. Description of the Related Art
Recently, as petroleum resources have decreased, not only various alternative energy sources, but also effective use of such energy sources, are required. In order to meet these requirements, cycle power generating systems using the combination of gas turbines and steam turbines, and cycle power generating systems using the combination of coal-gasification gas turbines and a steam turbines are being developed.
These cycle power generating systems each using the combination of a gas turbine and a steam turbine have a higher power generating efficiency than the conventional generating systems employing steam turbines operated by fossil fuels, and they are expected to be viable power generating systems for efficiently converting such fuels as natural gas and coal gas, whose production is expected to increase further, into electric power.
With the conventional gas turbine combustor used in such a gas turbine power generating system, the mixture of fuel and gas containing oxygen (generally air, and hereinafter referred to as "air") is ignited by a spark plug or the like and combusted uniformly. Generally, in this type of combustor, the fuel injected from a fuel nozzle into the inner tube of the combustor is mixed with air for combustion, fed under pressure from the air duct, ignited by the spark plug, and combusted. Cooling air and diluent air are added to the resultant gas, namely the combustion gas, in order to lower its temperature to a predetermined turbine inlet temperature. Thereafter, the thus-cooled and diluted combustion gas is injected through a turbine nozzle into a gas turbine.
One of the most serious problems which occurs in this conventional gas turbine combustor is that, a great deal of NOx is produced upon combustion of the fuel, thereby causing environmental pollution. This occurrence of NOx is attributed to the development of a localized high-temperature zone, the temperature of which exceeds 2,000.degree. C., in the combustor, during combustion of the fuel.
Various combustion systems are being investigated to solve the problem of the gas turbine combustor. For example, a catalytic combustion system using a solid phase catalyst has been proposed, in which such thin fuel as cannot be combusted in an ordinary combustor can be ignited. Therefore, with this system, the combustion temperature is not as high as the temperature which produces NOx and the turbine inlet temperature is as high as that of a conventional combustor.
The catalytic combustion type combustor has as a structural feature an auxiliary fuel injection nozzle and a catalyst body, arranged in series at the downstream side of the fuel injection nozzle with respect to the combusted gas flow passage. In general, the catalyst body has a honeycomb structure in which the mixture of fuel and air is combusted.
However, this catalytic combustion type combustor is also accompanied by the following problems. In the gas turbine, the temperature of the combustion gas to be injected into the turbine must be approximately 1,100.degree. C. and will tend to be much higher so that a higher efficient can be obtained. When the gas mixture is combusted at such a high temperature, however, the catalyst itself is heated to a temperature higher than 1,100.degree. C., with the result that the catalyst body tends to be broken. Through the experiments made by the inventors of this invention, it was confirmed that the temperature of the catalyst body was raised up to 1,100.degree..about.1,300.degree. C. In spite of this problem, a catalyst which withstands a temperature from 1,100.degree. to 1,300.degree. C. has not yet developed.
The inventors of this invention have proposed, as disclosed in U.S. Pat. No. 4,731,989, a catalytic combustion method which reduces the heat load exerted on a catalyst body by effectively utilizing the gas-phase combustion occurring on the downstream side of the catalyst body, in a combustor. According to this method, a diluent mixture gas of fuel and air is combusted at the catalyst body. Generally, when a diluent gas mixture which is not easily burnt is oxidized by using the catalytic body, contact combustion (catalytic combustion) on the surface of the catalyst occurs simultaneously with gas-phase combustion in the honeycomb structure. According to the above-mentioned method, however, the density, temperature and flow rate of the mixture gas are controlled such that only contact combustion occurs in the catalyst body. Since no gas phase combustion occurs in the catalyst body, the combustion temperature does not become high. Further, only part of the fuel is burnt, and combustion gas including the unburnt gas is exhausted from the catalyst body. As a result, the catalyst body can be prevented from being damaged by heat.
According to this proposal, new fuel, supplied from a fuel supply pipe provided downstream from the catalyst body, is added to the combustion gas exhausted from the catalyst body. Accordingly, the fuel density in the combustion gas is increased to induce gas-phase combustion on the downstream side of the catalyst body, thereby raising the temperature of the combustion gas to be supplied into the gas turbine. Normally, the gas-phase combustion on the downstream side of the catalyst body occurs at the thin mixing ratio side, to suppress the generation of NOx.
However, this proposed catalytic combustion method is faced with the following problem:
When high density fuel, which is not mixed with air, is delivered from the fuel supply pipe and added to the combustion gas exhausted from the catalyst body, the fuel density distribution in the combustion gas becomes uneven. In other words, an area of high fuel density and an area of low fuel density appear in the combustion gas on the downstream side of the catalyst body. Since the combustion temperature in the high fuel density area becomes higher than in the low fuel density area, a large amount of NOx is produced there.
For solving this problem, fuel supply means must be provided on the downstream side of the catalyst body so that the fuel density distribution becomes even. A means for supplying fuel from the interior of the combustor and a means for injecting fuel from the outside of the combustor are considered as such fuel supplying means.
The former means more easily equalizes the fuel density distribution than the later one. With the former means, however, the fuel supply means is exposed to gas at a high temperature, so that it is necessary to cool the fuel supply means. This causes the structure of the combustor to become complicated and lowers the reliability of the fuel supply means under high temperature. Thus, with the former means, the above problem has not yet been solved.
With the latter means, little trouble has arisen as to the heat resistance of the fuel supply means. However, a required fuel traveling distance must be obtained in order to ensure uniform fuel density distribution. The fuel traveling distance depends greatly on the fuel pressure. When the combustor is large, it is difficult to obtain the required fuel traveling distance under regular fuel pressure.